The present invention relates to a device and a method for fracturing a geological hydrocarbon reservoir, as well as a method of production of hydrocarbons.
In the production of hydrocarbons, the permeability and/or the porosity of the material constituting the reservoir have an influence on the production of hydrocarbons, in particular on the rate of production and thus the profitability. This is in particular what is referred to in the article Porosity and permeability of Eastern Devonian Shale gas” by Soeder, D. J., published in SPE Formation Evaluation, 1988, Vol. 3, No. 1, pp. 116-124, which describes the investigation of eight samples of Devonian shale gas, originating from the Appalachians. In particular, this article explains that the production of this shale gas presents the difficulty that the reservoir (i.e. the material constituting the reservoir) has low permeability.
Thus, various techniques exist for facilitating the rate of production of hydrocarbons, in particular from a reservoir of low permeability and of low porosity. These techniques consist of fracturing the reservoir statically or dynamically.
Static fracturing is a targeted dislocation of the reservoir, by injecting a fluid under very high pressure to crack the rock. Cracking is effected by a mechanical “stress” originating from hydraulic pressure obtained by means of a fluid injected under high pressure from a well drilled from the surface. It is also called “hydrofracturing” or “hydrosiliceous fracturing” (or else “frac jobs”, or more generally “fracking”, or “massive hydraulic fracturing”). Document US 2009/044945 A1 in particular presents a method of static fracturing as described above.
Static fracturing has the drawback that the fracturing of the reservoir is generally unidirectional. Thus, only the hydrocarbon present in the portion of the reservoir around a deep but highly localized crack is produced more quickly.
To obtain more diffuse fracturing, dynamic fracturing, or electrical fracturing, has been introduced. Electrical fracturing consists of generating an electric arc in a well drilled in the reservoir (typically the production well). The electric arc induces a pressure wave which damages the reservoir in all directions around the wave and thus increases its permeability.
Several documents discuss electrical fracturing. For example, document U.S. Pat. No. 4,074,758 presents a method consisting of generating an electro-hydraulic shock wave in a liquid in the wellbore to improve petroleum recovery. Document U.S. Pat. No. 4,164,978 suggests following the shock wave with an ultrasonic wave. Document U.S. Pat. No. 5,106,164 also describes a method of generating a plasma blast and thus fracturing a rock, but in the case of a borehole of small depth, for a mining application and not for production of hydrocarbons. Documents U.S. Pat. Nos. 4,651,311 and 4,706,228 present a device for generating an electric discharge with electrodes in a chamber containing an electrolyte, in which the electrodes are not subject to erosion by the plasma of the discharge. Document WO 2009/073475 describes a method of generating an acoustic wave in a fluid medium present in a well with a device comprising two electrodes between an upper packer and a lower packer defining a confined space. According to this document, the acoustic wave is maintained in a non-“shock wave” state in order to improve the damage, without however explaining the differences between “ordinary” acoustic wave and “shock” wave.
None of these documents produces entirely satisfactory fracturing of the reservoir. There is therefore a need for improved fracturing of a hydrocarbon reservoir.
For this, a method is proposed for fracturing a geological hydrocarbon reservoir, in which the method comprises a static fracturing of the reservoir by hydraulic pressure, and an electrical fracturing of the reservoir by generating an electric arc in a well drilled in the reservoir. According to the examples, the method can comprise one or more of the following features:                the static fracturing precedes the electrical fracturing;        the well is horizontal;        the electrical fracturing is repeated in various treatment zones along the well;        in each treatment zone, several arcs are generated in succession, preferably said arcs induce a pressure wave the rise time a rise time of which is decreasing;        in each treatment zone, the arcs are generated at a frequency equal to the resonance frequency of a material to be fractured in the reservoir;        the arcs are generated at a frequency below 100 Hz, preferably below 10 Hz, and/or above 0.001 Hz, preferably above 0.01 Hz;        the reservoir has a permeability below 10 microdarcy;        the reservoir is a shale gas reservoir.        the electrical fracturing is generated by a fracturing device which comprises two packers that between them define a confined space in a well drilled in the reservoir; a pump for increasing the pressure of a fluid in the confined space; an apparatus for heating the fluid; at least one pair of two electrodes arranged in the confined space; and an electric circuit for generating an electric arc between the two electrodes, the circuit comprising at least one voltage source connected to the electrodes and an inductance between the voltage source and one of the two electrodes;        the inductance is an adjustable inductance coil, preferably between 1 μH and 100 mH, more preferably between 10 μH and 1 mH;        the distance between the electrodes is adjustable, preferably between 0.2 and 5 cm, more preferably between 1 and 3 cm;        the voltage source comprises a capacitor with an adjustable capacitance;        the voltage source comprises a Marx generator.        
A method is also proposed for fracturing a geological hydrocarbon reservoir previously fractured statically by hydraulic pressure, in which said method comprises an electrical fracturing of the reservoir as described above. A method is also proposed for the production of hydrocarbons comprising the fracturing of a geological hydrocarbon reservoir by the method described above.